In the reprocessing of nuclear fuel materials, e.g. uranium oxide and thorium oxide fuel cores in pyrolytic graphite shells or sheaths, and like systems in which the nuclear fuel material is encapsulated or encased in a pyrolytic carbon sheath, fission-product inert gases are formed during insertion of the fuel elements into the reactor core in the so-called "head end" phase, the gases being trapped in the graphitic sheaths or the enclosures for the fuel elements. The fission-product inert gases are usually radioactive krypton-85, other krypton isotopes and xenon.
During the reprocessing stages, usually when the fuel element is solubilized in acid, a sparging gas stream is passed through the solubilizing unit and entrains the fission-product inert gases therewith.
The exhaust gases from the solubilizing unit thus contain radioactive components having a long half-life and which cannot be directly discharged into the atmosphere because of the environmental pollution hazard.
It is known to remove the radioactive components namely the fission-product inert gases, from such exhaust gases. In one published system (Whatley, M.E., "Calculations on the Performance of the KALC Process", ORNL-4859, 1973), the fission-product inert gases are washed out of the exhaust gas stream and absorbed by special solvents.
In another system for accomplishing a similar purpose, the fission-product inert gases are recovered by low temperature rectification of the exhaust gas stream (see BEAUJEAN H. et al, "Gaseous Radioactive Emissions from Reprocessing Plants and Their Possible Reduction", IAEA/SM-172/17, 1973).
It has also been proposed to use helium as a sparging gas in a closed chemical solubilizer and to recycle the helium thereto (see German Auslegeschrift DT-AS 24 26 764). The fission-product inert gases are removed from the helium before it is recycled by a sublimation or freezing process.
All of these prior art processes have the disadvantage that they are relatively costly and require complex and expensive apparatus and are not always technologically satisfactory. Furthermore, for the cleaning of the exhaust gas it is necessary usually to employ foreign media such as, for example, solvents like freons or sparging gases like helium.